1. Field of the Invention
The present invention relates to a filter circuit, a wireless transmitter, and a wireless receiver, and more particularly, to a filter circuit including a passive mixer.
2. Description of the Related Art
Generally, a wireless communication apparatus is equipped with a filter circuit for selecting a signal of a specific frequency (see, for example, Japanese Patent Application Laid-open No. 2005-130381). Such a filter circuit is built by connecting a parallel circuit constructed of an inductor and a capacitor to a transmission line. The resonant frequency f of the filter circuit is given by f=1/(2π(LC)1/2), where L [Henry] is the inductance of the inductor and C [Farad] is the capacitance of the capacitor.
If a frequency of a signal passing the transmission line is close to the resonant frequency, the impedance of the filter circuit becomes close to infinity, and as the frequency of the signal passing the transmission line goes apart from the resonant frequency, the impedance of the filter circuit decreases. Therefore, the filter circuit does not attenuate a signal having a frequency close to the resonant frequency, while attenuating a signal having a frequency apart from the resonant frequency, which makes it possible to select the signal having the frequency close to the resonant frequency.
When a variable inductor or a variable capacitor is used in the filter circuit, the resonant frequency can be changed by changing the inductance L or the capacitance, so that the signal attenuation band can be changed.
However, in a conventional filter circuit, because it becomes susceptible to an influence of the parasitic capacitance or the parasitic inductance as the resonant frequency (i.e., the filter center frequency) increases, it is hard to precisely match the resonant frequency to a target value by an electronic control, and it is hard to obtain a steep attenuation characteristic or a large attenuation amount.
Particularly when making the attenuation characteristic steep or increasing the attenuation amount, it is indispensible to precisely match the resonant frequency. However, with a frequency near 2 gigahertz, for example, it is hard to precisely match the resonant frequency on the order of 100 kilohertz.